High-temperature materials processing has become increasingly important as a technique for manufacturing many types of special performance industrial components. Precision shapes comprising a large variety of metallic and non-metallic materials selected for certain desirable performance characteristics are commonly manufactured using a high-temperature environment. For example, powder metallurgy is a familiar process for making a wide range of components and shapes from a variety of metals and alloys initially in powder form. The process utilizes pressure and heat to form the powders into precision shapes that require minimal secondary finishing.
In another familiar industrial application, one or a mixture of ceramic oxides and other ceramic-like compositions can be sintered to form a composite product of desired shape.. Although sintering occurs in loose powders, it is commonly enhanced by compacting the powder. In some applications, compacting is performed at room temperature, and the resulting compact is then sintered at elevated temperature without application of pressure. In other applications, a hot pressing process is used in which compacting is carried out at elevated temperature.
Typically, these and other types of high-temperature thermal processing operations are conducted in batch-style furnaces. More recently, there has been interest in continuous processing operations as a way to overcome some of the drawbacks of the batch furnace. Thus, U.S. Pat. No. 3,762,014 (Klein) describes a process for fabricating anode preforms by continuously sintering deposits of tantalum to a very thin tantalum foil strip utilizing a drive apparatus which is adjacent to but completely external of the furnace. The Klein apparatus comprises a continuous belt driven by rotating drums to carry the foil strip and deposits into a furnace maintained at a temperature of 1800.degree. C.-2500.degree. C. The belt speed is adjusted so that a given section of the foil strip remains inside the furnace sufficiently long (e.g. 1-60 minutes) for the sintering to be completed. While inside the furnace, the foil strip is supported on a stationary, horizontally-disposed, refractory support member (reference numeral 40 in FIG. 4 of the Klein patent). The continuous belt (reference numeral 31 in FIG. 4 of the Klein patent) never enters the sintering furnace (reference numeral 37).
A different approach to continuous sintering utilizes a belt furnace design in which articles to be heated, such as discrete shaped containers holding metal or ceramic powder, are placed on a continuous conveyer belt to be carried into, through and out of a furnace preheated to appropriate temperature. In general, such a belt furnace design for sintering silicon nitride is described in an article by Dale E. Wittmer et al. entitled "Continuous and Batch Sintering of Silicon Nitride" appearing in the American Ceramic Society Bulletin, vol. 72, no. 6 (June 1993) at pages 129-137, and in a second article by Dale E. Wittmer et al. entitled "Comparison of Continuous Sintering to Batch Sintering of Si.sub.3 N.sub.4 " appearing in Ceramic Bulletin, vol. 70, no. 9 (1991), both of which are incorporated herein by reference. U.S. patent application Ser. No. 08/541,711, now U.S. Pat. No. 5,648,042, of which this application is a continuation-in-part, is directed to an improved, multi-zone continuous belt-type furnace apparatus utilizing interlocking links of sintered silicon carbide especially designed for such high-temperature processing applications, and that disclosure is incorporated herein by reference.
In sintering, powder metallurgy and similar materials processing, articles and components of various shapes are produced by agglomeration of fine powders, typically under heat and pressure. These techniques are commonly employed where other methods of forming or shaping such articles, for example casting, forging and machining, are impractical or where special material properties are required. Sintering temperature is typically somewhat below the melting point of the powder. Depending on the material and required processing conditions, various techniques can be utilized to hold the particulates together in a desired shape while heat and/or pressure are applied.
Thus, for some applications, metallic powders or mixtures of metallic and nonmetallic powders are shaped by cold pressing at room temperature between steel dies or mold sections, which produces initial adhesion and shaping of the particles. In one common form of cold pressing, the shaping/compaction step is followed by heating the compacted particles to a suitable temperature in a nonoxidizing atmosphere, while retaining the article in the dies, to obtain final cohesion through sintering. The utility of this procedure is limited, however, to processing particulates which can be sintered at temperature below those at which the steel or other die material begins to soften and melt. Thus, this procedure cannot be used to sinter particulates such as various ceramic oxides, silicon nitride, and the like which require sintering temperatures well above the melting point of steel.
Regardless of the shape forming methods employed (i.e., cold pressed, extruded, injection molded, slip cast, thixotropic cast, gel cast, tape cast, or pressure cast), the potential for warpage of the part being formed during the sintering process exists if the part is not supported in some manner during the sintering operation. For simple geometries (e.g., rods, cylinders, tubes), the parts are conventionally held in v-grooves or semi-circular cradles during the sintering process. Parts with more complex geometries, however, cannot be sintered in a horizontal position with such fixtures. Attempts to use fixtures similar to those used for simple geometries for supporting shapes with complex geometries will result in undesirable shape distortion during sintering due to shrinkage and gravity effects. Typically parts with regular cross-sections but more complex geometries (e.g., flaired tubes, closed-end tubes and valves) are supported vertically on center rods or suspended from a collar or keeper. In this manner gravity aids in keeping the part straight during the densification which accompanies sintering. Even with this type of supporting system, however, warpage and part distortion can occur due to uneven heating of the parts. For parts with irregular cross-sections, these vertical support techniques are even less satisfactory.
These and other problems with and limitations of the prior art are overcome with the high-temperature flowable sintering bath of this invention.